1. Field of the Invention
The present invention relates to a signal processing apparatus, a signal processing method, and a reception system. More particularly, the invention relates to a signal processing apparatus, a signal processing method, and a reception system for estimating, rapidly and with required precision, errors of the carrier used illustratively to demodulate the OFDM (Orthogonal Frequency Division Multiplexing) signal.
2. Description of the Related Art
Terrestrial digital broadcasts and like broadcasting schemes adopt OFDM (Orthogonal Frequency Division Multiplexing) as their data (i.e., signal) modulation method.
Under OFDM, numerous orthogonal subcarriers are provided within the transmission band, each subcarrier being assigned data in its amplitude and phase for digital modulation such as PSK (Phase Shift Keying) and QAM (Quadrature Amplitude Modulation).
According to OFDM, the transmission band is divided into a large number of subcarriers. It means that for each subcarrier, the bandwidth is narrow and the modulation rate is low. However, the total transmission speed (of all subcarriers) is the same as that of ordinary modulation methods.
Since data is allocated to a plurality of subcarriers under OFDM as mentioned above, data modulation may be carried out by IFFT (Inverse Fast Fourier Transform) computation. The OFDM signal resulting from the modulation may be demodulated by FFT (Fast Fourier Transform) computation.
It follows that the transmission apparatus for transmitting the OFDM signal may be constituted using circuits performing IFFT computations and that the reception apparatus for receiving the OFDM signal may be formed using circuits effecting FFT computations.
Under OFDM, signal segments called guard intervals are provided to improve resistance to multipath interference. Also according to OFDM, pilot signals (i.e., signals known to the reception apparatus) are inserted discretely in the direction of time as well as in the direction of frequency. These pilot signals are used by the reception apparatus for synchronization and for estimating transmission channel characteristics.
Because of its high resistance to multipath interference, OFDM is adopted notably by terrestrial digital broadcasts that are vulnerable to the effects of such multipath interference. The terrestrial digital broadcasting standards adopting OFDM illustratively include DVB-T (Digital Video Broadcasting-Terrestrial) and ISDB-T (Integrated Services Digital Broadcasting-Terrestrial).
Under OFDM, data is transmitted in units called OFDM symbols.
Generally, the OFDM symbol is formed by effective symbols that constitute a signal period in which IFFT is performed in modulation, and by a guard interval made by a partial waveform of the latter half of the effective symbols being copied unmodified to the beginning of the effective symbols.
The guard interval attached to the beginning of the OFDM symbol helps enhance resistance to multipath interference.
The terrestrial digital broadcasting standards adopting OFDM define the unit called a frame (OFDM transmission frame) made up of a plurality of OFDM symbols. Data is then transmitted in units of frames.
The reception apparatus for receiving the above-described OFDM signal uses an OFDM signal carrier to effect digital orthogonal demodulation of the OFDM signal.
Generally, however, the OFDM signal carrier used by the reception apparatus for digital orthogonal demodulation does not coincide with the OFDM signal carrier employed by the transmission apparatus transmitting the OFDM signal; the carrier contains errors. That is, the frequency of the OFDM signal used for digital orthogonal demodulation is shifted from the center frequency of the OFDM signal (i.e., its IF (Intermediate Frequency) signal) received by the reception apparatus.
For that reason, the reception apparatus is arranged to perform two processes: a carrier shift amount detection process for detecting a carrier shift amount which is the error of the OFDM signal carrier used for digital orthogonal demodulation, and a correction process (i.e., offset correction) for correcting the OFDM signal in such a manner as to eliminate the carrier shift amount.
Meanwhile, DVB-T2 (Digital Video Broadcasting-Second Generation Terrestrial in Europe) is being worked out as a terrestrial digital broadcasting standard that adopts OFDM.
The so-called BlueBook (DVB BlueBook A122) describes DVB-T2 (“Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2),” DVB Document A122, June 2008; called Non-Patent Document 1 hereunder).
Under DVB-T2 (as set forth in BlueBook), a frame called the T2 frame is defined. Data is transmitted in units of T2 frames.
A T2 frame (representing the OFDM signal) includes two preamble signals called P1 and P2 that contain information necessary for processes such as OFDM signal demodulation.
FIG. 1 is a schematic view showing the format of a T2 frame. The T2 frame contains P1 symbols, P2 symbols, and data symbols, in that order.
The P1 symbols are symbols for transmitting P1 signaling including a transmission type and basic transmission parameters.
More specifically, the P1 signaling (P1) symbols include parameters S1 and S2 indicating whether P2 symbols are transmitted by the SISO (Single Input Single Output (meaning one transmitting and one receiving antenna)) method or by the MISO (Multiple Input, Single Output (meaning multiple transmitting antennas but one receiving antenna)) method. The parameters also indicate the FFT size (i.e., number of samples (symbols) subject to a single FFT computation) for performing FFT computation of P2.
It follows that to demodulate P2 may require demodulating P1 beforehand.
The P2 symbols are symbols for transmitting L1 pre-signaling and L1 post-signaling.
The L1 pre-signaling includes information for allowing the reception apparatus receiving T2 frames to receive and decode the L1 post-signaling. The L1 post-signaling includes parameters required by the reception apparatus in gaining access to the physical layer (i.e., to the physical layer pipes).
One T2 frame may have P2 of 1 to 16 OFDM symbols disposed therein.
The P1 and P2 symbols include known pilot signals. Specifically, pilot signals of P1 are disposed on the subcarriers that are not periodically located, whereas pilot signals of P2 are disposed on the subcarriers that are periodically located. Of the pilot signals, those located periodically at intervals of a predetermined number of subcarriers (symbols) are called SP (Scattered Pilot) signals; the pilot signals disposed on the subcarriers of the same frequency are called CP (Continual Pilot) signals.
Also, the reception apparatus performs FFT computation of the OFDM signal per OFDM symbol. DVB-T2 defines six FFT sizes, 1 K, 2 K, 4 K, 8 K, 16 K, and 32 K, each FFT size being the number of symbols (subcarriers) making up one OFDM symbol.
The spacing between the subcarriers of OFDM symbols (i.e., subcarrier spacing) is inversely proportional to the FFT size of the OFDM symbol. Thus under DVB-T2, defining the FFT size is equivalent to stipulating the subcarrier spacing.
DVB-T2 also stipulates that, of the six above-mentioned FFT sizes, 1 K should be used for the OFDM symbols of P1. It is further stipulated that for P2 and other OFDM symbols other than P1, any one of the six FFT sizes above may be used.
It follows that regarding the OFDM symbols of P1, solely the subcarriers having the widest subcarrier spacing (corresponding to the FFT size of 1 K) defined by DVB-T2 are used.
With regard to P2 and other OFDM symbols other than P1, that is, OFDM symbol of P2 and OFDM symbol of data (Normal), it is possible to use not only the subcarriers having the widest subcarrier spacing defined by DVB-T2, but also the subcarriers having a subcarrier spacing other than the widest subcarrier spacing (i.e., any one of the spacings corresponding to the FFT sizes of 2 K, 4 K, 8 K, 16 K, and 32 K).
FIG. 2 is a schematic view showing an OFDM signal of P1.
The OFDM signal of P1 has 1 K (=1024) symbols as its effective symbols.
This signal has a cyclic structure in which a starting part A1 of the effective symbols A is frequency-shifted to a signal C copied ahead of the effective symbols and the remaining part A2 of the effective symbols A is frequency-shifted to a signal B copied behind the effective symbols.
The OFDM signal of P1 has 853 subcarriers as its effective subcarriers. Under DVB-T2, information is located on 384 subcarriers out of the 853 subcarriers.
The implementation guidelines of DVB-T2 (ETSI TR 102 831: IG) say that if the transmission band for transmitting the OFDM signal is illustratively 8 MHz, then the correlation of information location on the above-mentioned 384 subcarriers regarding the OFDM signal of P1 may be used to estimate a “coarse” carrier frequency offset with accuracy of up to ±500 kHz.
The implementation guidelines also say that in the case of P1, the cyclic structure explained above in reference to FIG. 2 may be used to estimate a “fine” carrier frequency offset with accuracy of up to ±0.5 multiplied by the subcarrier spacing.
DVB-T2 stipulates that the FFT size of P1 should be 1 K samples (symbols) as mentioned above.
DVB-T2 also stipulates that if the transmission band is illustratively 8 MHz, then the sampling period for P1 with the FFT size of 1 K samples should be 7/64 μs.
Thus when the transmission band is illustratively 8 MHz, the effective symbol length Tu of P1 is 1024×7/64 μs.
Meanwhile, there exists the relation defined by the expression D=1/Tu, where Tu (in seconds) denotes the length of the effective symbols out of the OFDM symbols (i.e., effective symbol length excluding guard intervals), and D (in Hz) represents the subcarrier spacing of the subcarriers of the OFDM signal.
Thus if the transmission band is 8 MHz, the subcarrier spacing D of the subcarriers of P1 is approximately 8,929 Hz, which is the inverse of the effective symbol length Tu=1024×7/64 μs.
As described, because the subcarrier spacing D of P1 is about 8,929 Hz, the “fine” carrier shift amount can be estimated using P1 with an accuracy of ±8,929/2 Hz.
In this case, the capture range of P1, i.e., the range into which the carrier of the OFDM signal used for digital orthogonal demodulation may be captured by correcting the OFDM signal in keeping with the “fine” carrier shift amount obtained from P1, is within ±8,929/2 Hz (between −8929/2 Hz and +8929/2 Hz) in reference to the true value of the OFDM signal carrier.
Given the carrier shift amount estimated using P1 with the FFT size of 1 K, it is possible to capture the carrier of the OFDM symbols having the FFT size of 1 K into the range of ±0.5×subcarrier spacing D, whereby the OFDM symbols are demodulated.